Mach: speed of sound at what altitude ?
n0r...@summa.tamu.edu
this refers to 2.2 times the speed of sound at what altitude ?
Could someone explain ? Thanks in advance...
-RN
Albion H. Bowers
That is at the altitude at which it reaches that speed. Typically a speed
like that will be reached at like 40,000 ft. Let me give you a generic
example:
a **********
l **** ***|
t ** |
i * |
t * #
u * ####
d * ######
e * ###
* #
0 mach number --->
Apologies for the ugly ASCI drawing. but, the * symbols will indicate the
lift limit (this assumes a constant G level, usually quoted for 1 G, but it
could be any G level). Next the | symbols will indicate a Mach limit,
sometimes due to inlet compressor face total temp limit, or a turbine temp
limit, or (more common on modern fighters) the canopy temp limit ("the
speed of heat"). In fact the F-4 had a little thermo couple on the canopy
and you could go as fast as you like as long as you could, but then you'd
get a temp warning pretty quickly. And the # symbols could be for dynamic
pressure limits, or inlet duct pressure limits, or whatever.
That's a really simple case, many times there are many other constraints
that squeeze in on an aircraft's operating envelop.
Hope that helps...
Al Bowers
--
Al Bowers DOD #900 Alfa Ducati Hobie Kottke 'blad Iaido
National Aeronautics and Space Administration Dryden Flight Research Facility
Lead Aero F-18 HARV Chief Engineer SR-71 work: bow...@rigel.dfrf.nasa.gov
"Take up an attitude with the sun behind you..." -Miyamoto Musashi
P Atcliffe
Mach 2+ will ALWAYS be at high altitude (say, 30,000 feet and above), where
the local speed of sound is around 660 mph. I don't know what the current
low altitude speed record is, but I reckon it won't be much more than about
Mach 1.3. FYI, Mach 1 at sea level is roughly 760 mph or 660 knots.
Mike Campbell
Mach values are relative to the speed of sound at any given altitude.
From memory Mach 1 = 760 mph at standard atmosphere values at sea
level, and 660 mph @ 30,000 feet.
Thus Mach 2 is 1520 mph at sea level, but only 1320 mph @ 30,000 ft.
Mike Campbell, Christchurch, New Zealand
mi...@aloysius.equinox.gen.nz
Mary Shafer
Mike> In article <14JUL199...@summa.tamu.edu>
Mike> n0r...@summa.tamu.edu writes:
>
> When an aircraft is referred to as being capable of flying at
> (say) Mach 2.2, this refers to 2.2 times the speed of sound at
> what altitude ? Could someone explain ? Thanks in advance...
Mike> Mach values are relative to the speed of sound at any given
Mike> altitude. From memory Mach 1 = 760 mph at standard atmosphere
Mike> values at sea level, and 660 mph @ 30,000 feet.
Mike> Thus Mach 2 is 1520 mph at sea level, but only 1320 mph @ 30,000
Mike> ft.
The Mach 3+ SR-71 won't even go supersonic at sea level. The maximum
Mach number is used in these descriptions.
If you plot Mach number versus altitude for a plane's flight envelope,
you get kind of a lozenge shape at about 45 deg. Usually it runs out
the zero-altitude line to some dynamic pressure and then follows that
line of constant dynamic pressure up to some Mach limit. Then it goes
at constant Mach up to the maximum altitude. Going along the other
side of the figure, you arc up the minimum control speed to the
maximum altitude and then over to the max Mach number.
Notice that the dynamic pressure is a function of altitude just like
Mach number. q-bar = 1/2 rho V**2 where rho is the density.
--
Mary Shafer DoD #362 KotFR NASA Dryden Flight Research Facility, Edwards, CA
sha...@ferhino.dfrf.nasa.gov Of course I don't speak for NASA
"A MiG at your six is better than no MiG at all." Unknown US fighter pilot
Jacquelin Aldridge
>-RN
It could mean any altitude depending on the planes max ceiling, the thing
is you can travel at the same speed at any altitude it's just a matter of
how much power your expending to maintain that altitude.
The higher you go the faster you can go for the same amount of
power expended.Thats why they make planes like the SR-71 go so high so it
can as fast as possible and use the minnimum amount of fuel there by
increasing there range.
Jeff Crowell
: >n0r...@summa.tamu.edu writes:
: >-RN
Hmmm. Okay, here's the deal (sound like someone we all know?)
Mach Number refers to the speed of the aircraft (or missile or whatever),
divided by the speed of sound at that altitude, the altitude that the
plane is at at that moment. The result is a decimal number.
Speed of sound changes with many factors, chief among them temperature
(at least for air).
'Indicated Airspeed' refers to how many air molecules pile up in the
pitot tube, compared to the pile in static air (which is why they call it
a pitot-static system). Wings see indicated airspeed, which is why the
speed gauge of an airplane uses IAS (well, okay, actually Calibrated
Airspeed, CAS, because that's corrected for instrument error)--that way,
you're always aware of where you are in relation to stalling speed, a
piece of info always close to a pilot's heart (or maybe other places).
"True Airspeed' refers to the rate of motion of the airplane (or ...)
with respect to the ground. It is IAS, corrected for instrument error
and density factors (temperature, altitude, phase of the moon...).
TAS changes rapidly with altitude. An aircraft can climb at a constant
IAS and its actual rate of motion (TAS) will increase as it climbs.
Mach number is calculated using True Airspeed (TAS).
I'm told that at SR-71 altitudes, the air molecules are several feet
apart. Never been there, myself. But it means that TAS is a lot more
than IAS--and local speed of sound is a lot lower than at sea level. So
it's easier to get high Mach numbers at higher altitudes. And it's less
likely your plane will melt out from under you, which I understand an
F-15 can do if it goes too fast at lower altitudes.
Jeff
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Old soldiers never die. Young ones do.
William J Levenson
[good stuff deleted]
>
>"True Airspeed' refers to the rate of motion of the airplane (or ...)
>with respect to the ground. It is IAS, corrected for instrument error
>and density factors (temperature, altitude, phase of the moon...).
>TAS changes rapidly with altitude. An aircraft can climb at a constant
>IAS and its actual rate of motion (TAS) will increase as it climbs.
A small nit...True airspeed is not relative to the ground but, rather, to
the air in which the aircraft is flying which is probably in motion itself
(i.e. wind). "Ground speed" is speed relative to the ground.
Just wanted to avoid confusion
Bill Levenson
Mary Shafer
WJL> In article <CAEzL...@boi.hp.com> jc...@boi.hp.com (Jeff
WJL> Crowell) writes:
>"True Airspeed' refers to the rate of motion of the airplane (or ...)
>with respect to the ground. It is IAS, corrected for instrument
>error and density factors (temperature, altitude, phase of the
>moon...). TAS changes rapidly with altitude. An aircraft can climb
>at a constant IAS and its actual rate of motion (TAS) will increase
>as it climbs.
WJL> A small nit...True airspeed is not relative to the ground but,
WJL> rather, to the air in which the aircraft is flying which is
WJL> probably in motion itself (i.e. wind). "Ground speed" is speed
WJL> relative to the ground.
Actually, your correction doesn't address the basic problem with the
first posting. The "true" in true airspeed means that the airspeed
has been corrected for compressibility effects. It's actually corrected
calibrated airspeed, not corrected indicated airspeed.
Indicated airspeed is just what you read off your pitot-static system.
It may be corrected for temperature. The indicated airspeed may be
corrected for the position of the probe, if the probe is inside the
flow over the airplane. This resulting airspeed in calibrated
airspeed. In faster aircraft, it's important to compensate for
compressibility effects and the result of this is true airpeed. The
correction for compressibility is a function of airspeed as well as
altitude.
Below is an excerpt from a posting I wrote some time ago on how to
instrument your own research airplane, which discusses some of the
issues here.
AIR DATA, ANGLE OF ATTACK AND SIDESLIP
Probably the single most important set of data is the air data. This
comprises altitude, air speed, Mach number, dynamic pressure, and
relative wind. Every researcher wants to know these.
Boom-mounted pitot-static systems
The most common way to get research air data is to mount a
pitot-static system, with temperature probe, on a long, rigid boom on the
nose of the aircraft. On this same boom is mounted angle of attack
and sideslip vanes. The reason for using the boom is to put these
sensors out in the free stream, where the flow is not yet disturbed by
the local flow fields over the aircraft.
I call it a rigid boom, but it probably isn't, so corrections for boom
bending sometimes have to be made, as do upwash and sidewash
corrections if the vanes are in the flow field.
As an example, the F-8 Digital Fly-by-Wire had such a boom and the
sideslip vane was just inside the shock wave at 1.35 Mach. This
produced a little 10 Hz, 0.1 deg oscillation on the signal. It took
me a while to figure this out, since the angle of attack signal was
just sitting there, really steady, but I finally walked down to the
aircraft, stared at the boom, and had it dawn on me. Fortunately,
this was eminently ignorable, so we didn't have to move the vane.
If the program is going to look at high angles of attack, where the
nose vortices have an extremely large effect on the dynamics of the
aircraft, a nose-mounted boom can't be used. One might then go to a
boom mounted on a wing tip, although this isn't as good because it's
frequently inside the flow field. Plus, it's more likely to have
dynamics of its own, with a cantilevered, somewhat flexible boom out
on the end of a cantilevered, somewhat flexible wing.
Calibration of the pitot-static instruments is accomplished with a
pneumatic test cart. It's plugged into the ports and pumped up to
known pressures.
The angle of attack and sideslip vanes are calibrated with a
protractor that fastens onto the boom. The vane is then moved
manually.
The Space Shuttle had a pop-out boom, known by a rather vulgar
nickname, during the flight test phase (the first five flights). It
had to be protected during the hot part of re-entry and was deployed
somewhere around Mach 1 or 2.
Flush Air Data Systems
If a boom isn't appropriate, a Flush Air Data System (FADS) can be
used. This is an array of pressure ports on the nose of the aircraft.
There are quite a few of these ports (20+). The port pointing
directly into the flow provides total pressure (i.e. the pitot port)
and a reference port provides ambient pressure (i.e. the static port).
The pressure pattern also provides angle of attack and sideslip.
The Space Shuttle version is called SEADS, or Shuttle Entry Air Data
System. SEADS, on Columbia, is a vertical line of 8 ports and a
horizontal line of 6 more ports, in a cross.
The individual pressure ports are calibrated using the pneumatic test
cart.
System Calibration
The sensors are calibrated on the ground, but the proof of the air
data system comes in flight. What you're looking for are the
corrections for position error, local flow, etc. There are a _lot_ of
ways to calibrate them, but the most common are:
1. Pacer aircraft--just like it sounds, a calibrated airplane
flies beside the test vehicle and, when they're both in straight
and level, unaccelerated flight, pacer airspeed and altitude are
read out. This is time-consuming, but very accurate.
2. Tower fly-by--the test vehicle flies straight and level,
unaccelerated, by the fly-by tower. The airplane is photographed
against a grid and the data is read up to determine airspeed and
altitude. The calibration is limited to the on-the-deck flight
envelope, so that you can't usually calibrate out to Mach 2, for
example. Also, turbulence, for example, can be a problem because
the test aircraft is right on the deck.
3. Trailing cones and bombs--a calibrated cone or bomb can be
trailed behind the test vehicle. The trail position puts the cone
down into the free stream. Works best for slow aircraft. Once
I saw an airplane flying over Dryden, trailing a cone. At first I
thought it had a _tiny_ chase plane ....
4. Inertial navigation system, ground radar, global positioning
system, etc. These need weather balloon data for winds aloft, so
that ground speed can be translated into airspeed. Flying a measured
ground track works the same way. The ground track goes in a square
so that winds aloft are cancelled out (in theory).
When the FADS was calibrated, it was a multiple-step process. First the
nose-boom system was calibrated. Then a wing-mounted boom system was
calibrated against the nose-boom system. The nose-boom system was then
removed and the FADS installed in the nose of the vehicle. Finally the
FADS was calibrated against the wing-mounted system. The wing-mounted
system was then removed so that nothing would interfere with the flow.
The INS was also used in this process, for confirmation.
Many airplanes also have a production pitot-static system, mounted on
the fuselage. These are just fine to fly the aircraft by, but not
usually good enough for flight test. They are sometimes deep inside
the flow field of the aircraft.
Many aircraft also have radar altimeters which are very accurate and
useful, but only fairly close to the ground, of course. The Gossamer
Albatross that we tested here at Dryden had a sonar altimeter, from a
Polaroid SX-70 camera.
True airspeed can also be determined using a laser velocimeter, but I
don't think that this is a common piece of flight test instrumentation
yet.
Standard Atmosphere
All altitudes, airspeeds, Mach numbers are corrected to the Standard
Day, so that data from one flight can be compared to all other flights.
This also provides a means of comparison with wind tunnel data.
Jerre M Hill
is for density changes due to changes in pressure and temperature
at altitude. Compressibility effects are generally neglible
for low speed-aircraft (Mach < .3) but the density effect can
be quite large.
Jerre Hill
Mechanical Engineering Dept
UNC-Charlotte
Charlotte, NC
Jeff Crowell
: In article <CAEzL...@boi.hp.com> jc...@boi.hp.com (Jeff Crowell) writes:
: >"True Airspeed' refers to the rate of motion of the airplane (or ...)
: >with respect to the ground.
: A small nit...True airspeed is not relative to the ground but, rather, to
: the air in which the aircraft is flying which is probably in motion itself
: (i.e. wind). "Ground speed" is speed relative to the ground.
: Just wanted to avoid confusion
: Bill Levenson
Mea maxima culpa, you are absolutely correct. WRT ground is groundspeed
(duh). I got carried away by it all!
Jeff
###################################################################
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# Jeff Crowell | | #
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# _________| _( ) _|_________ #
# DMD Process Engineer +/ _| |( . )| |_ \+ #
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The only acceptable substitute for knowledge is silence.
Paul Raveling
>
> Mach values are relative to the speed of sound at any given altitude.
> From memory Mach 1 = 760 mph at standard atmosphere values at sea
> level, and 660 mph @ 30,000 feet.
Actually the direct relation is to air temperature,
but almost everything flies in the altitude zone where
temp is mostly a function of altitude. The ones who notice
the difference most are pilots of U-2's and related models,
whose cruise speed range at altitude is a spread of 5 to 6 knots
sandwiched between stall buffet and Mach buffet. They
wind up flying at constant Mach and variable altitude
to suit air temperature changes.
------------------
Paul Raveling
Rave...@Unify.com
Bob Luchetti
>
>[good stuff deleted]
>>
>>"True Airspeed' refers to the rate of motion of the airplane (or ...)
>>with respect to the ground. It is IAS, corrected for instrument error
>>and density factors (temperature, altitude, phase of the moon...).
>>TAS changes rapidly with altitude. An aircraft can climb at a constant
>>IAS and its actual rate of motion (TAS) will increase as it climbs.
>
>(i.e. wind). "Ground speed" is speed relative to the ground.
>
>Just wanted to avoid confusion
>
>Bill Levenson
>
To be totally Specific, the Formula ICE-T will give you TAS
Indicated Airspeed is what you read on the dial. Calibrated is Indicated
corrected for installation and position errors of the Pitot-static system.
Equivelant AS includes a correction for compressability (only significant
in high speed A/C to include airliners). The 3 of these corrections
together will provide True Airspeed
:
Bob Luchetti
INTERNET: 72310.1176<at>COMPUSERVE.COM
Edwards AFB, CA - "Best Test in the West"
"Gravity isn't easy, but it's the law"